Uranyl Extraction by TBP from a Nitric Aqueous Solution to SC-CO2: Molecular Dynamics Simulations of Phase Demixing and Interfacial Systems

In relation with the liquid-liquid extraction of uranyl nitrate from an acidic aqueous phase to supercritical CO2, we present a series of molecular dynamics (MD) simulations on the "interfacial" systems involving UO2(NO3)2 species and high concentrations of TBP and nitric acid. We compare the distribution of solvent and solutes at the interface which forms upon the demixing of "chaotic mixtures" of water / CO2 solutions. The simulations highlight the importance of interfacial phenomena in uranyl extraction to CO2. In most cases, demixing leads to separation of aqueous and CO2 phases which form an interface. At low concentrations, TBP and the neutral form HNO3 of the acid adsorb at the interface, while the uranyl salt and ionic species sit in water. Spontaneous complexation of uranyl salts by TBP is observed, leading to UO2(NO3)2(TBP)(H2O) and UO2(NO3)2(TBP)2 species of 1:1 and 1:2 stoichiometry, respectively, which adsorb at the interface. As the TBP concentration is increased, the proportion of 1:2 species, more hydrophobic than the 1:1 species, increases, following the Le Chatelier principle. Nitric acid competes with the uranyl complexation by TBP which forms hydrogen bonds with H3O+ or HNO3 species. Thus, at high acid.TBP ratio, the concentration of 1:1 and 1:2 complexes decreases. Also noteworthy is the evolution of the interface from a well-defined border at low acid and TBP concentrations, to a mixed microscopic "third phase" containing some 1:2 complexes which can be considered as "extracted". We believe that such heterogeneous microphase is important for the stabilization and extraction of uranyl complexes by TBP and, more generally, in the extraction of highly hydrophilic cations (e.g. lanthanides or actinides) to organic media.